The present invention relates generally to a method of determining tissue viability. More specifically, the present invention relates to a non-invasive method of determining tissue viability using visible and near-infrared spectroscopy.
In surgery, the ability to determine whether or not a tissue will survive is of paramount importance. This is particularly true in plastic surgery where efforts to develop a reliable method to predict skin flap viability are ongoing. It is therefore not surprising that efforts to develop a reliable method to predict tissue viability go back to the very beginnings of plastic surgery. Clinical assessment, based on observations of color, temperature and capillary perfusion have always been, and remain, the basis of good management. However, the clinical signs which arise as a consequence of poor blood perfusion along the flap become evident only after several hours of compromised perfusion. Prolonged and severe deprivation of oxygen and other nutrients to the tissues results in irreversible tissue damage leading to necrosis and loss of tissue. Early, nonsubjective detection of poor tissue oxygenation following surgery increases the likelihood that intervention aimed at saving the tissue will be successful. As a consequence, a variety of methods have evolved over the years to augment clinical judgement. Broadly speaking, these methods fall into two categories: those that are based on an assessment of blood flow within tissues and those that examine the cellular metabolism within tissues.
Early studies depended on the use of pharmacological agents to assess blood flow (Hynes, 1948, Br J Plast Surg 1:159-171; Conway et al, 1951, Surg Gynecol Obstet 93:185-189). It was soon realized that the use of pharmacologic agents gave inconsistent results which were difficult to interpret.
Later, vital dye studies and the use of radioisotopes added to our knowledge (Dingwall and Lord, 1943, Bull Johns Hopkins Hosp 73:129-134; Kety, 1949, Am Heart J 38:321-328). Also, fluorescein dye or other labelled tracer measurements of blood flow were necessarily invasive procedures and the invasiveness of the procedure coupled with prolonged washout times limited the frequency with which these methods could be applied to the site of interest.
More recently, Doppler evaluation of blood flow (Swartz et al, 1988, Plast Reconst Surg 81:149-161), assessment of oxygen transport (Hjortdal et al, 1990, Scand J Plast Reconstr Surg Hand Surg 24:27-30) and sophisticated monitoring of tissue temperature (Kaufman et al, 1987, Ann Plast Surg 19:34-41) held promise. Laser Doppler velocimetry had the advantage of being a non-invasive method. However, it only measured the blood flow within a small volume of tissue, and microvascular heterogeneity rendered the method prone to errors: motion of the probe during measurement and poor probe placement reproducibility both led to poor reproducibility. In addition, the recognition of arteriovenous shunting, where blood flow bypasses the capillary bed resulting in non-nutrient flow, reduced the value of a strictly blood flow-based investigation. Blood flow methods primarily address the issue of oxygen delivery, but provide little information on cellular utilization.
Finally, the most recent advances examine the metabolic status of tissues using magnetic resonance imaging (Cheung et al, 1994, Magn Reson Med 32:572-578) and infrared spectroscopy (Irwin et al, 1995, Br J Plast Surg 48:14-22). Magnetic resonance spectroscopy can provide information on the metabolic status of skin flaps at a cellular level (Cheung, 1994), but the extended measurement times, high costs, and limited portability render the method clinically impractical.
Regarding infrared spectroscopy, U.S. Pat. No. 3,638,640 to Shaw, for example, teaches disposing radiation sources and detectors about the ear of a patient and measuring the intensity of radiation passing therethrough. The logarithms of the detector responses are then combined linearly to yield an indication of oxygen saturation in the blood based on the ratio of concentration of oxyhemoglobin to total hemoglobin in the patient""s ear. Clearly, this device is limited in that it provides information only on the oxygenation level of the blood.
U.S. Pat. Nos. 4,223,680 and 4,281,645, both to Jxc3x6bsis, teach a method and art apparatus for continuous in vivo monitoring of metabolism in a body organ using near infrared spectroscopy. Specifically, oxygen sufficiency in an organ is measured based on the absorbance characteristics of cytochrome a, a3. However, this apparatus is arranged to measure changes and trends in metabolism and not tissue viability.
U.S. Pat. No. 5,074,306 to Green teaches a method for measuring burn depth comprising administering a fluorescent compound to the skin burn and exciting the fluorescent compound with infrared light. The amount of fluorescence detected at the skin burn compared to the adjacent unburned skin provides an indication as to the depth of the burn. However, as with fluorescein dye and other vital dyes discussed above, this method is invasive and may harm the compromised tissue or the surrounding tissue. Furthermore, the device is used to determine the depth of the burn in the tissue, not the viability of the tissue.
U.S. Pat. No. 5,497,770 to Morcos teaches an apparatus for measuring dynamic oxidative metabolism in compromised myocardium. Specifically, Morcos teaches injecting one or more metabolic substrates in a region of the compromised myocardium and collecting near infrared spectra over time using a probe inserted into the region of interest. The collected data is then used to determine whether a metabolic pathway cascade or a transmembrane ionic potential are intact, which is in turn used to provide an indication as to the viability of the cells. Clearly, this method is too invasive for use with damaged tissue, as it requires both the insertion of a probe below the surface of the tissue and injection of one or more metabolic substrates into the tissue.
Irwin (Irwin, 1995) teaches a method and a device for continuously monitoring concentration changes in oxyhemoglobin, deoxyhemoglobin and total hemoglobin. However, the method is limited in that the device must be mounted onto the surface of the tissue being examined and, as a result, is prone to interference if the probe is removed and subsequently replaced on the suspect tissue. Furthermore, as noted above, the method and the device are designed to measure trends meaning that hemoglobin levels must be continuously monitored in order for useful data to be collected.
It is therefore not surprising that a recent survey amongst microsurgeons showed a lack of uniformity in the use of monitoring devices to assess viability of transplanted tissue (Neligan, 1993, Microsurgery 14:162-164). The ideal test to predict skin viability should be quick and simple to perform, accurate and reproducible, inexpensive, non-invasive, cause little or no patient discomfort and should not alter the basic physiology of the tissue.
According to the invention there is provided a method of assessing tissue viability of a patient comprising:
accessing a portion of tissue of the patient wherein the communication of fluids between the portion and a main body of the patient is compromised such that the viability of the tissue portion, that is, survival or outcome of the tissue portion, is in question;
locating a probe at the tissue portion;
collecting from the probe at least one spectrum of visible and near infra-red light from the tissue portion;
analyzing the spectrum to generate data related to the viability of the tissue portion, wherein viability is related to the outcome of the tissue portion in the absence of any intervention aimed at altering said outcome;
and using the data to make a determination of the viability of the tissue portion, wherein sufficient spectral points and data are collected so that the determination of the viability of the tissue may be made from a single spectrum. As a result, an indication as to tissue viability is immediately available, without the need to take multiple readings and analyze trends. This in turn means that corrective measures can be taken more quickly, thereby improving the likelihood of tissue survival.
The tissue portion may form a portion of the body which is damaged. The damage may be caused by burning or freezing.
The tissue portion may be attached to the main body portion by rejoining severed blood vessels, thereby compromising the communication of fluid from the main body portion to the tissue portion.
Preferably, the data related to the viability of the tissue portion comprises data on oxygenation of the tissue portion. The data on the oxygenation of the tissue portion may comprise comparing levels of deoxyhemoglobin and oxyhemoglobin. The levels of deoxyhemoglobin and oxyhemoglobin may be compared by measuring the ratio of absorbance at a wavelength between 790-810 nm to absorbance at a wavelength between 740-780 nm. More specifically, the levels of deoxyhemoglobin and oxyhemoglobin may be compared by measuring the ratio of absorbance at 800 nm to absorbance at 760 nm.
The method may include the steps of removing the probe, subsequently relocating the probe proximal to the tissue portion, collecting from the probe at least one spectrum of visible and near infra-red light from the tissue portion, analyzing the spectrum to generate data related to the viability of the tissue portion and comparing the data from the first analysis with the data from the subsequent analysis. Thus, the above-described method may be used to examine tissue viability over time without the need for continuous monitoring.
The method may include the steps of removing the probe, subsequently relocating the probe proximal to a second tissue portion, collecting from the probe at least one spectrum of visible and near infra-red light from the second tissue portion, analyzing the spectrum to generate data related to the viability of the second tissue portion and comparing the data from the tissue portion with the data from the second tissue portion. As the above-described method does not require the monitoring of trends, the probe can be removed from the patient and used to examine another tissue portion.
The second tissue portion may be of a second patient.
Preferably, the comparison of the data involves comparing the spectra at at least one reference point. The reference point may comprise a wavelength at which the absorption coefficient of deoxyhemoglobin and oxyhemoglobin is approximately equal, for example, at a wavelength between 790-810 nm. The wavelength may be 800 nm.
Preferably, the data related to the viability of the tissue portion comprises data on hydration of the tissue portion. The data on the hydration of the tissue portion may be obtained by measuring the water content of the tissue portion, for example, by measuring the ratio of absorbance at a wavelength between 960-1080 nm to absorbance at 900 nm. The ratio of absorbance may be taken at 980 nm to 900 nm.
Preferably, the data related to the viability of the tissue portion comprises data on both hydration and oxygenation of the tissue portion. The data on the oxygenation of the tissue may be obtained by measuring the ratio of absorbance at a wavelength between 790-810 nm to absorbance at a wavelength between 740-780 nm and the data on the hydration of the tissue portion may be obtained by measuring the ratio of absorbance at a wavelength between 900-1080 nm to absorbance at 900 nm. The data on the oxygenation of the tissue may be obtained by measuring the ratio of absorbance at 800 nm to absorbance at 760 nm and the data on the hydration of the tissue portion may be obtained by measuring the ratio of absorbance at 980 nm to absorbance at 900 nm.
The probe may be located at least 1 mm away from the tissue portion. That is, the probe does not have to be mounted or attached to the damaged tissue to collect spectrum data.
Preferably, the spectrum is collected in the absence of added metabolic substrate.